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\IBVShead{5912}{23 November 2009}

\IBVStitle{2007 Photometry of UV Leonis}

\IBVSauth{HECKERT, PAUL A.}

\IBVSinsto{Dept. of Chem. \& Physics, Western Carolina University, Cullowhee, NC 28723 USA}

\SIMBADobj{UV Leo}

\begintext

\vspace*{-1cm}

UV Leo is a member of the short period eclipsing group of RS CVn systems.
McCluskey (1966) performed an early photometric study and summarized earlier
work on UV Leo. Frederik and Etzel (1996) performed a complete optical
photometric study of this system.

Kjurkchieva et al. (2007) performed an optical photometric and spectroscopic
study of this system. Their photometry was on the nights of April 4, 14, and
15, 2007. For this work, I collected new optical photometry of UV Leo in
2007. This new photometry, collected about a month later, can tell us
something about how rapidly the spots on UV~Leo evolve.

I observed UV Leo with the San Diego State University 61-cm telescope on Mt.
Laguna. The light curves were obtained on the nights of May 8, 9, 11, 13,
16, 23, \& 28, 2007. I used SAO 99225 as the comparison star and SAO 99223
as the check. Using standards of Landolt (1983), I calibrated the comparison
star magnitudes: $B=9.26, V=8.20, \hbox{R=7.61}$, and $I=7.11$. The calibrated check
star magnitudes are: $B=8.77, V=8.33, R=8.04$, and $I=7.77$. The complete four
filter light curves, with 120 data points per filter, are plotted in Figure
1. The data are differential magnitudes (var-comp) in the standard
Johnson-Cousins system. I used the ephemeris of McClusky (1966):
$$\phi _{0}=2438440.7275+0.6000855 E.$$

Figure 1 shows considerable variations in the out of eclipse portion of the
light curve between phases 0.6 and 0.9. To check if these variations are
scatter in the data or changes in the light curve, Figure 2 shows the V band
data divided into three groups. Group 1 includes the data from the first
four nights, May 8, 9, 11, and 13. Group 2 includes the data from May 16,
and Group 3 includes the data from May 23 and 28. It is apparent that the
out of eclipse portions of the light curve changed during the nearly three
weeks between the beginning and end of the observations.

Because the period is so close to 0.6 days the same phases are available to
observe in a three night repeating cycle. It was therefore not possible to
fill in the small gaps in the light curves that occurred when the phase at
the start of a night was a little after the phase at the end of a previous
night. Hence removing the later nights in Group 3 only slightly affects the
phases covered, however the photometry is less dense. To completely cover
the light curve, it was also necessary to observe at a higher than optimal
air mass at the ends of the nights. Therefore the last few data points for
each night have more scatter than most of the data for the night. These data
are at phases are 0.80, 0.47, 0.57, 0.97, and 0.11. The data deviating most
from the models in the clean fits (See Figure 4.) were at these phases.
However removing these data would have made the light curves to be modeled
even more sparse.

I modeled the 90 data points from Groups 1 and 2 using Budding and Zeilik's
(1987) Information Limit Optimization Technique (ILOT). Initial values for
stellar parameters were in most cases taken from Kjurkchieva et al. (2007).
I used $k(=r_{2}/r_{1})=0.9668$, $r_{1}=0.30$, $i=84\fdeg$2, $q(=m_{2}/m_{1})=0.954$, $T_{1}=6000$, and $T_{2}=5970$. I adopted the limb darkening
coefficients for each wavelength from Frederik and Etzel (1996). After the
initial fit, the ILOT extracts a distortion wave which I then, in an
iterative procedure, fit for two circular 3400K spots. The fits for each
color are performed independently. The reported longitude, latitude, and
radius of each spot are in degrees. The latitude is the most difficult spot
parameter to fit. Attempts to fit the spot latitudes produced errors that
were in some cases larger than the possible range of latitudes. The
preliminary fits however yielded mid range latitude values, so I fixed the
spot latitudes at 45\deg\ for the final spot models. Figure 3 shows the $V$
band spot fit. For the spot fits I get:

\begin{table}[!hr]
\vspace*{-0.2cm}
\centerline{Spot Fits}
\begin{center}
\begin{tabular}{lcccc}
\hline
2007 & $B$ band & $V$ band & $R$ band & $I$ band \\ \hline
Longitude$_{1}$ & 260.9$\pm $3.3 & 260.0$\pm $4.1 & 258.4$\pm $4.2 & 255.9$%
\pm $4.6 \\ 
Radius$_{1}$ & 17.4$\pm $0.9 & 16.6$\pm $1.0 & 17.7$\pm $1.0 & 17.3$\pm $1.1
\\ 
Longitude$_{2}$ & 47.8$\pm $7.6 & 47.4$\pm $9.4 & 41.2$\pm $7.7 & 40.7$\pm $%
8.4 \\ 
Radius$_{2}$ & 9.8$\pm $1.5 & 9.4$\pm $1.7 & 12.1$\pm $1.5 & 12.4$\pm $1.6
\\ 
$\chi ^{2}$ & 234.4 & 105.6 & 189.9 & 192.8 \\ \hline
\end{tabular}
\end{center}
\vspace*{-0.5cm}
\end{table}

Kjurkchieva et al. (2007) find two spots near the equator at longitudes of 0\deg\ and 110\deg\ and radii of 27\deg\ and 24\deg. Because my data were
taken only about a month after the Kjurkchieva et al. (2007) data, comparing
the spot models tells us that the spots on UV~Leo can changed considerably
in this short time.  

It should be noted that they used Binarymaker 3.0 to model their data, and
this code uses a different convention for latitude and longitude than the
ILOT code. The ILOT code measures latitude north and south from the equator
and longitude from phase 0.0 increasing with phase. The Binarymaker code
uses the Wilson-Devinney code convention. Latitude is measured south from
the north pole, and longitude on the primary star is measured from the
primary eclipse increasing in the direction of orbital motion. So 
longitude(Binmaker) = 360\deg - longitude(ILOT) and latitude(Binmaker) 
= 90\deg - latitude(ILOT).

The latitude comparison is not definitive because I used a fixed latitude.

To definitively compare the spot longitudes, one should note that I used the
original McClusky (1966) ephemeris, while Kjurkchieva et al. (2007) cite an
updated ephemeris. The ILOT clean fits, after correcting for the spots,
compute the best fit for the phase correction correction needed so that the
primary and secondary eclipses occur at phases 0.0 and 0.5. For these light
curves, this correction is $-7\fdeg 1$. Therefore the longitudes in the above
table should be reduced by $7\fdeg 1$ for direct comparison to the longitudes
reported by Kjurkchieva et al. (2007). Hence the Kjurkchieva et al. (2007)
spot at 110\deg\ longitude is at 250\deg\ longitude in the ILOT convention
and is very nearly the same longitude as the 253\deg\ longitude of my spot
corrected as above. The other spot however migrated nearly 40\deg\ from the
primary eclipse in about a month. Hence one of the spot longitudes changed
significantly.  

The spots also became smaller in size, and at the same time cooler, during 
the month between the two sets of light curves.  

The ILOT can estimate spot temperatures by comparing infrared models to
visual models. Using the initial values of 0K spot parameters at a visual
wavelength, I fit infrared data for the unit of light and flux ratio. The
value of the spot temperature can be found from the flux ratio of the star's
photospheric temperature and the spot temperature. Comparing the $R$ to $V$ data
did not give a valid fit. I compared the $R$ to $B$, $I$ to $V$, and $I$ to $B$ fits.
The reported spot temperature is the average of these three comparisons.
Doing so I find an average value of the spot temperature of $T_{s}=3400\pm300$K. In an iterative procedure, I then fit the other spot parameters to
3400K spots. In this second iteration the spot longitudes and sizes differed
from the values found in the first iteration by less than the reported
errors. Hence, I made no further iterations. The spot parameters in the
table above are for 3400K spots.

After the spot fits, I performed clean fits to the light curves removing the
effects of the distortion wave from the spot as modeled in that filter. I
fit each wavelength independently and averaged the color independent
parameters. The mass ratio, q, is difficult to determine photometrically, so
I fixed this parameter at q=0.954, the value found spectroscopically by
Kjurkchieva et al. (2007). For the other color independent parameters, I
get: $k(=r_{2}/r_{1})=0.920\pm0.007$, $r_{1}=0.291\pm0.002$, and
$i=83\fdeg4\pm0\fdeg2$. Figure 4 shows the $V$ band clean fits.

My value for the inclination, $i=83\fdeg4\pm0\fdeg2$, is between the
values found by Kjurkchieva et al. (2007), 84\fdeg2, and Frederik and
Etzel (1996), 82\fdeg6.

The ratio of the radii, $k(=r_{2}/r_{1})$, provides the most
astrophysically interesting comparison with previous results. In this work,
I get $k=0.920$. Frederik and Etzel (1996) find $k=1.097$, so that the secondary
star is larger, even though less massive, than the primary. Kjurkchieva et
al. (2007), on the other hand, find that $k=0.967\pm0.064$ $(r_{1}=0.30\pm0.01$ and $r_{2}=0.29\pm0.01$), which agrees to within the errors
with my result. In any case the two components of this system are close to
the same radius and mass.

This work shows that the spot structure on UV Leo can evolve on time scales
of a few weeks. The modeled stellar parameters are consistent with previous
work.

\medskip

I thank Paul Etzel for scheduling generous amounts of observing time at Mt.
Laguna. I also acknowledge support from Western Carolina University and the
American Astronomical Society Small Research Grant Program.

\bigskip

\IBVS2fig{7.89cm}{5912-f1.eps}{BVRI light curves of UV Leo}{5912-f2.eps}{UV Leo V data grouped by date}
\IBVSfigKey{5912-f1.eps}{UV Leo}{light curve}
\IBVSfigKey{5912-f2.eps}{UV Leo}{light curve}

\clearpage

\IBVSfig{7.6cm}{5912-f3.eps}{UV Leo V spot fit}
\IBVSfigKey{5912-f3.eps}{UV Leo}{spot light curve}


\IBVSfig{7.6cm}{5912-f4.eps}{UV Leo V clean fit}
\IBVSfigKey{5912-f4.eps}{UV Leo}{model light curve}


\references

Budding, E. and Zeilik, M., 1987, {\it Astrophys. J.}, {\bf 319}, 827

Frederik, M.C. and Etzel, P.B., 1996, {\it Astron. J}., {\bf 111}, 2081

Kjurkchieva, D.P., Marchev, D.V., 2007, {\it MNRAS.}, {\bf 381}, 663

Landolt, A.U., 1983, {\it Astron. J.}, {\bf 88}, 439

McCluskey, Jr., G.E., 1966, {\it Astron. J}., {\bf 71}, 536

\endreferences

\IBVSedata{5912-t2.txt}
\IBVSdataKey{5912-t2.txt}{UV Leo}{photometry}

\end{document}
